1. Technical Field
The embodiments herein generally relate to radio frequency (RF) technologies, and, more particularly, to RF downconverters used with complementary metal oxide semiconductor (CMOS) technologies.
2. Description of the Related Art
Achieving a low cost RF solution generally mandates the use of a direct conversion architecture, such as the one illustrated in FIG. 1, to eliminate the image reject RF surface acoustic wave (SAW) and the intermediate frequency (IF) SAW. In this case, with the use of relatively inexpensive and non-complex silicon technologies such as CMOS, the design of a high linearity RF downconversion mixer becomes very difficult in the existence of large adjacent blocking signals in addition to the sensitivity problems occurring due to flicker noise upconversion. Moreover, achieving a low cost highly linear low noise front-end generally becomes very difficult in a power conscious environment such as wireless receivers.
In other words, achieving highly linear RF downconverters in technologies such as CMOS is generally quite challenging for low power applications. One major challenge is the noise level degradation due to the flicker noise of the CMOS mixer. To solve this problem, CMOS passive mixers 4 are often used in a RF system 1 since they contribute much less flicker noise because their DC current is equal to zero. The RF system 1 shown in FIG. 1 further includes an antenna 2 adapted to feed RF signals into a low noise amplifier (LNA) 3 and a pair of low pass filters 5 connected to the CMOS passive mixers 4. However, the use of CMOS passive mixers 4 typically results in extra challenges. A regular CMOS mixer typically switches the RF signal in the voltage domain across a low frequency impedance (typically resistive).
In order to achieve a proper operation, two RF buffers 16, 17 may be used after the LNA 13 and prior to the mixers 14 as shown in the RF system 11 of FIG. 2. Generally, the role of these buffers 16, 17 are: (1) splitting the signal to the in-phase (I) and quadrature (Q) paths; (2) providing a well-defined RF equivalent low impedance voltage reference prior to the passive mixer (i.e., the output resistance of the buffer should be very low). Preferably, the buffers 16, 17 have a very wide dynamic range. In other words, its noise preferably is very low and its linearity should be very high given that it comes after the high gain LNA 13. Similar to the RF system 1 of FIG. 1, the RF system 11 of FIG. 2 further includes an antenna 12 and a pair of low pass filters 15.
Generally, all of the above requirements translate to very high power consumption in the RF buffers 16, 17. To achieve a performance comparable to its expensive bipolar-based technologies counterparts, the CMOS front-end will tend to consume a very high current. Accordingly, a clear disadvantage of the conventional solutions is very high current consumption. Wideband RF LNAs are typically required in advanced RF applications. However, the use of wideband resistive feedback amplifiers tend to result in a poor noise figure (NF). As such, other techniques should be investigated.